The present invention relates to improved low density resin compositions and to their manufacture.
The various marine and sub-marine techniques are known to have recourse to materials having particular properties, particularly a low density, a high resistance to hydrostatic pressure, a high resistance to hydrolysis and a low absorption capacity for water under pressure.
The prior art has proposed, under the generic name of "syntactic foams", a number of materials consisting essentially of a charge of hollow spheres distributed within a resin matrix, according to the ASTM definition.
As a rule, the charge of hollow spheres to be introduced into the syntactic foams may be of inorganic or organic nature; microspheres of borosilicate glass or silica of usual diameter from 10 to 200 microns are widely used. The density of the foam may be further decreased by adding macrospheres of glass or of thermoplastic or thermosetted resins, optionally reinforced with fibers, their diameter being, for example, from 2 to 50 mm; however such a macrosphere addition is often detrimental to the compressive strength.
Widely used resins for manufacturing syntactic foams of the prior art are thermosetting resins, such as polyepoxides or polyesters, whose density is higher than 1.1 g/cm.sup.3.
The use of thermosetting resin compositions based on polybutadiene optionally diluted with a monomer polymerizable in the presence of free radical has also been disclosed in the prior art; it has the advantage of using a resin of density close to 0.9 g/cm.sup.3. The process disclosed in U.S. Pat. No. 3,856,721 has thus recourse to 1,3-butadiene homopolymers or copolymers of the latter with a styrene compound; they are defined as containing at least 40% by weight of butadiene, at least 80% thereof being butadiene units of the 1,2 type.
Other compositions of syntactic foams based on polybutadiene have also been disclosed by the applicant, for example, in French Pat. No. 2,346,403, corresponding to U.S. Pat. No. 4,107,134; these compositions are defined as obtained from polybutadienes with 50 to 80% of 1,2-units; these polymers may be optionally partially epoxidized or silanized, to thereby improve the sticking of the resin on the glass spheres; the resin may be optionally reinforced by further addition of a fiber charge.
French Pat. No. 2,361,438 discloses using a resin based on at least one non-functional polybutadiene of very high 1,2-units content and a bi- or poly-functional polybutadiene of lower 1,2-units content, together with a polyfunctional organic compound to lengthen the chains of the functional polymer by polycondensation, the whole being optionally diluted with a vinyl monomer to be polymerized in the presence of free radicals.
The latter compositions, based on polybutadiene resins, according to the prior art, had the advantage of a better control when curing the syntactic foam, particularly by reducing the exotherm in the foam. This problem is particularly critical when manufacturing articles of large size and section from syntactic foam, such as certain marine floats.